Hot pressing barrier material



3,145,103 HOT PRESSING BARRHER MATERIAL Robert G. Carlson, Greenhills, ()hto, assignor to the United States of America as represented by the Secretary of the Air Force No Drawing. Filed Aug. 23, 1963, Ser. No. 304,289 2 Claims. or. 75-214 This invention relates to a barrier material for use in minimizing the migration of raphite into the powdered material being pressed into objects of manufacture by powder metallurgy methods.

Powder metallurgy employs heat and pressure applied to finely divided metal powders for making useful articles. The process temperature commonly is slightly below the fusion point of the principal constitutent of the powdered metal. Bonding between the metal particles depends on the interdiifusion of the metal particles in the solid state. In the sintering process the formation of oxide films is minimized by means of a vacuum, an atmosphere that is reducing or that is inert.

Powder metallurgy involves the plastic deformation of metal particles in article products fabrication, to accurate dimensions and contour with little or no finishing operations. The process extends to cermets, which are mix-. tures of ceramics and metals and to refractory materials.

The hot-pressing of powders of the refractory metals are at such high temperatures that the usual methods of melting and casting are not applicable. The hot-pressing of refractory metal powders have as limiting aspects, relatively short die life and where used, carbon contamination from graphite dies or molds.

In pressing refractory materials at temperatures above 3000 F., which corresponds to 1650 0., die materials are restricted to high melting point materials of which graphite is representative. The objectionable ingress of carbon from the graphite die material into the refractory metal that is being compacted by the hot-pressing processes into the end products, results.

Niobium Nb (or columbium Cb) melts at 2500:50" C. (or 4532" F.). Tantalum Ta, melts at 2996:50" C. (or 5425 F.). Tungsten W, melts at 3370 C. (or 6098 F.).

The present invention provides carbon-free end products made of iobium, tantalum, tungsten and mixtures thereof, refractory metals and the like, that are produced by the hot-pressing of these materials in graphite dies with a sheet of tantalum barrier material interposed between the powdered metal and the graphite die. The sheet of tantalum is of adequate thickness and uniformity of composition and purity, such that it completely blocks the migration of carbon from the graphite die into the hot-pressed powdered metal within the die, even though the sheet of tantalum may be quite thin, such for example at .005 inch in thickness.

The effectiveness of the tantalum barrier that restrains the carbon of the graphite die from its migration into the powdered and compressed metal, is dependent upon the 3,l45,l3 Patented Aug. 18, 196% absence of any yield stress of the tantalum at the temperature and the pressure to which it is subjected.

Experimentally, hot-pressings were made on four types of columbium or niobium particles that were pure and that were alloyed with tungsten. The four types were: (1) columbium spheres of from 30 to 35 mesh; (2) colurnbium fine powder of -200 mesh; (3) columbium powder of equal quantities of 200 mesh and +350 mesh; and (4) prealloyed by weight 50% tungsten and 50% columbium powder of equal quantities of 200 mesh and +350 mesh alloy.

The above four samples of particles were separately pressed in the temperature range of between 2750 F. and 3825 E, which temperatures correspond respectively to 1510 C. and 2107 C., and at presures up to 4000 p.s.i., in graphite molds or dies.

In all of these samples the tantalum sheet was found to be an excellent carbon barrier during the compaction of the columbium and columbium with tungsten alloy particles, at the stipulated temperatures and pressures.

In these experiments no significant differences in the densification rates between the static and the fluctuating or hydraulic press methods of pressure application were observed and all of the samples produced were free of added carbon in their surface structure.

The life of the graphite dies that are used in this type of operation is materially increased by the use of the tantalum sheet between the die cavity and the powdered metal that is compressed into the die cavity.

It is to be understood that the specific materials that are stipulated herein are experimentally confirmed for their fineness at the temperatures and the pressures to which they were subjected and that similarly operating modifications may be made therein within the scope of the present invention.

I claim:

1. The process of hot-pressing powdered refractory metal in graphite die by interposing a carbon barrier of tantalum therebetween, and thereby minimizing the migration of graphite from the die to the metal during the hot pressing of the powdered refractory metal.

2. The process of hot pressing powdered refractory metal selected from the group that consists of niobium, tantalum and tungsten in a graphite die by placing a sheet of tantalum against the graphite in the graphite die, placing the powdered refractory metal against the sheet of tantalum and out of contact with the graphite die, and applying heat and pressure to the powdered refractory metal in causing its conversion to a solidified and a unified product that is free from carbon contamination from the die.

Pfanstiehl Nov. 11, 1919 Schell et al. Dec. 15, 1959 

1. THE PROCESS OF HOT-PRESSING POWDERED REFACTORY METAL IN GRAPHITE DIE BY INTERPOSING A CARBON BARRIER OF TANTALUM THEREBETWEEN, AND THEREBY MINIMIZING THE MIGRATION OF GRAPHITE FROMTHE DIE TO THE METAL DURING THE HOT PRESSING OF THE POWDERED REFRACTORY METAL. 